Fatigue & Lifetime

Understanding the Next Step After Reaching DWP Reaching the Design Working Period does not automatically mean that a crane must be removed from service. Instead, it signals that the original design assumptions regarding fatigue and duty have been fulfilled. At this stage, continued operation requires structured evaluation rather than routine maintenance alone. This is where […]

Why Lifetime Is Evaluated Differently When discussing the Design Working Period, it is important to distinguish between two fundamentally different components of a crane: The load-bearing structure The hoisting mechanism Although they operate together, their lifetime evaluation is based on different principles. Structural Lifetime: Cycle-Based Assessment The crane structure, including: Main girders End carriages Load-bearing

Cycles and Load Spectrum in DWP Assessment The lifetime of a crane structure is governed by more than just its rated capacity. Design Working Period is fundamentally influenced by two key parameters: The number of operating cycles The load spectrum during those cycles Both factors must be considered together to understand cumulative structural demand. Number

The Difference Between Years in Service and Structural Usage It is common to describe cranes as “10-year-old” or “20-year-old” equipment.However, calendar age alone does not determine whether a crane is close to the end of its safe working life. In reality, structural lifetime depends on how intensively the crane has been used. Time in Service

Understanding the Real Lifetime of a Crane A crane is not designed for unlimited use.Its structural components and mechanisms are engineered for a finite working life, defined not by calendar years, but by operational usage. This concept is known as the Design Working Period (DWP). Lifetime Is Measured in Cycles, Not Years It is common

Real shafts are never perfectly smooth cylinders. They contain geometric discontinuities such as: Shoulders Fillets Keyways Grooves Threads These features introduce local stress amplification known as stress concentration. In fatigue design, correctly accounting for this effect is critical. Theoretical Stress Concentration Factor Kt The theoretical stress concentration factor is defined as: Kt = σmax σnom

Why 95% and 99.9% Reliability Change Your Shaft Size Fatigue strength is not a single deterministic value. It is a statistical result derived from experimental testing. Even under identical loading conditions, different specimens of the same material will fail at different numbers of cycles. This scatter makes reliability a fundamental parameter in fatigue design. Statistical

The endurance limit obtained from laboratory testing does not represent real engineering conditions. Standard S-N data are generated using: Polished specimens Small diameters Controlled laboratory environments Fully reversed loading In practice, real components differ significantly from these ideal conditions. Therefore, the laboratory endurance limit S′e must be corrected before being used in design calculations. From

Fatigue analysis is not a single method but a family of approaches developed for different damage mechanisms and life regimes. Three major fatigue-life methods are commonly used in engineering design: Stress-Life (S-N) Method Strain-Life (ε-N) Method Linear Elastic Fracture Mechanics (LEFM) Although all three address fatigue failure, they are based on fundamentally different assumptions and

In practical shaft design, loading is rarely completely reversed. Most components operate under fluctuating stresses with a non-zero mean stress. The classical S-N diagram, however, is generated under the condition: σm = 0 This creates a fundamental limitation. When mean stress is present, the alternating stress must be corrected before it can be used with